1. Field of the Invention
The present invention generally relates to a multiplex transmission system. More particularly, the invention relates to a multiplex transmission system of the type in which a plurality of units of electrical equipment and appliances disposed at various points of a vehicle, such as an air conditioner, a cruising controller, an intermittently operating wiper, and a variety of lamps, are connected loop-like by at least one wiring harness and besides the signal transmission and reception thereamong are effected in a multiplex manner through the wiring harness.
2. Description of Relevant Art
Recently, with an increase in the number of units of electrical equipment and appliances to be disposed at various points of a vehicle, the wiring harness for interconnecting such equipment and appliances tends to have an increased number of wires and hence to be enlarged in size, thus lowering the design feasibility, productivity, fittability, and maintainability of the wiring harness, while raising indirect costs such as for control and storage. In conjunction with such problems, there is a growing tendency to employ a multiplex transmission system in which a plurality of multiplex harness control devices (hereinafter called "nodes") are connected loop-like by at least one wiring harness and the signal transmission and reception thereamong is effected in a multiplex manner. Some examples of such multiplex transmission systems for vehicles are disclosed in "Jidosha Gitjutsu (Automobile Technology)" Vol. 38, No. 2, published 1984, at pages 215 to 221.
This reference includes comments on a number of types of multiple transmission systems, as follows.
According to the reference, multiplex transmission systems are classified into several types by the system of multiplexing, control, communication, and synchronization.
As multiplexing systems, there are enumerated a time division system and a wavelength division system, besides others; the former being suitable for multiplexing those transmission systems in which a wiring harness is employed as a signal transmission line, and the latter, for those in which an optical fiber is employed as a signal transmission line.
As control systems, typically there are two: a centralized control system in which principally a single node is responsible for the control of an entire system, and a distributed control system in which the control of a system is equally shared among a plurality of nodes. However, as a hybrid of typical systems, there may well exist a variety of control systems that are unable to be fairly classified into either of the typical two categories.
As communication systems, generally there are two: a parallel line system in which a signal of address and a signal of detection and control data are transmitted through separated lines, and a serial line system in which such signals are transmitted through a single line. In either system, there is additionally provided a power line with which respective nodes are connected loop-like.
The serial line system usually appears as either a message system in which an address signal and a detection and control data signal are combined to be transmitted as a single message, or a time-slot system in which an address signal is omitted while transmitting a series of detection and control data signals in a predetermined sequence.
As synchronization systems, there are employed two: an internal clock system in which respective nodes have clocks thereof, and an external clock system in which a clock signal is generated at a particular node and supplied therefrom to other nodes. In the latter system, the particular clock generator that is adapted to generate and supply the clock signal is used to govern the other nodes. Thus, this system may well be regarded as a variation of the centralized control system.
In this respect, in the centralized system, an entire system may be shut down with troubles at a master node. As a control system for the multiplex transmission system, therefore, the distributed control system is peferred to be employed, though in this case there is a desideratum to be attained by devising a communication system to avoid confusion of signal transmission among respective nodes.
Incidentally, when performing control of a load as an electrical equipment or appliance, it sometimes becomes necessary to execute a logical process in addition to on-off control of a single switch. Exemplarily, in the case of a vehicle, there are adduced reasons such as that some loads are required to have their patterns of action selectable depending on various positions of an ignition switch, such as "on", "off", "park", and "lock" positions and some loads such as a tail lamp are needed to be operable for on-off control from a plurality of switches such as a tail switch and a parking switch.
For such reasons, in general, in a time-division multiplex transmission system according to a distributed control system, each node is constituted with: (1) a combination of a transmitter and a receiver connected both to a wiring harness; (2) a detection circuit such as a sensor or a switch put under control of the node; (3) a drive circuit such as a relay or a power switch for starting a load such as a pump or a lamp put under control of the node; (4) a logic circuit connected to the detection circuit and the drive circuit; (5) a combination of a multiplexer and a demultiplexer for interconnecting the logic circuit with the transmitter and the receiver; and (6) a control circuit connected to, to cooperate with, the multiplexer and the demultiplexer and adapted for signal transmission to and reception from the wiring harness.
In the accompanying drawings, FIG. 8 is a schematic representation of a time-division multiplex transmission system for vehicles of such a type as suggested from the level of art described.
In FIG. 8, designated by reference character 200 is the multiplex transmission system, which employs a distributed control system as the control system, a message type serial line system as the communication system, and an internal clock system as the synchronization system.
The multiplex transmission system 200 is constituted with five ordinary nodes 201, 202, 203, 204, and 205 each respectively having such circuitries as (1)-(6) above, and a bidirectional bilateral signal transmission path (wiring harness) consisting of a pair of transmission lines 211, 212 with which the nodes 201 to 205 are connected loop-like.
As a starting method for multiplex transmission systems of such constitution, there is a generally employed system in which: among repective associated nodes, a given one is selected in advance as a temporary master node; and the entire system begins to operate in harmony with the master node.
The node 201 is now supposed to be a temporary master node.
Then, there may well be employed a method in which, after application of power, the node 201 is first let to rise, that is, caused to start transmitting an address signal and a data signal, to thereby bring the entirety of the multiplex transmission system 200 into a raised state. Thereafter, the respective nodes are to be adapted to function for transmitting and receiving address and data signals, one after another, in a cyclic manner.
However, according to such system raising method, in the case where the node 201 as a temporary master node has failed to rise for some reason, the remaining nodes 202 to 205 are indefinitely left as they are waiting rise of the node 201, thus keeping the entirety of the transmission system 200 from exhibiting functions thereof.
Further, even if the temporary master node 201 has successfully risen after power application and hence the entirety of the system 200 has been started, it may so happen in employment of certain transmission line arrangements that, due to disconnection at either of the transmission lines 211, 212, the signal transmission and reception cycle by the nodes 201 to 205 will not advance.
Such state may occur under such a condition as well that, while the system 200 is operating, any node happens to malfunction.
As a conclusion, in respect of the distributed control system as applied to a time-division multiplex transmission system, there is a desideratum to be achieved to attempt to eliminate the possibility of malfunction of the entire system.
On the other hand, in the multiplex transmission system 200, it is necessary to execute a data check.
In this respect, in general, without limiting the description to multiplex transmission systems, as a checking method of an output signal on a signal transmission path there is employed a system in which check codes or data for error detection are added to the end of data to be transmitted; exemplarily, there being added such codes as horizontal and vertical parity check codes and a cyclic redundancy check code. Such check codes are used for checking whether the signal as output to the transmission path is varied due to effects such as noise from outside. According to such check system, a remarkably accurate data check is enabled by use of various check codes.
However, according to such data check systems, even when a signal transmitted from a certain node, which signal consists of an address signal, a data signal, and a check code, is varied by noises before signal reception at another node, the content of the signal as received at the latter node may be judged to be correct, because of an unexpected variation of the check code. If the data check is incorrect, various functions of the system itself may be unsuccessfully exhibited.
As a conclusion, in respect of the control system as applied to a multiplex transmission system, there is a desideratum to be achieved to permit the presentation of a more favorable data check system.
The present invention has been achieved to effectively solve such problems and substantiate such desiderata as described in conjunction with a conventional multiplex transmission system.